Lifetime and toughness are required for a rolling bearing. Particularly, the balance of both properties is important to a rolling bearing for use in applications where a high load or an impact load is imposed, such as wind power generator, construction machine, iron-steel plant and industrial robot.
The rolling lifetime of a bearing depends on reasons which are roughly classified into internally originating-type flaking and surface originating-type flaking. The internally originating-type flaking originates from a nonmetallic inclusion contained in a steel material, and therefore, a technique of reducing the oxygen amount in a steel material is employed for prolonging the lifetime. The oxygen amount has been heretofore reduced by various improvements of the iron and steel process, and a large carbon amount in chemical components is known to be desirable for reducing the oxygen amount, and a bearing steel represented by SUJ2 exhibits high cleanliness, compared with S53C that is a medium-carbon steel.
In the surface originating-type flaking, flaking occurs due to stress concentration on the edge of an indentation produced by the engagement with a foreign substance such as metal powder contained in oil, and for the purpose of relieving the stress concentration, the retained austenite amount is controlled to thereby prolong the lifetime. In general, the surface originating-type flaking apparently shortens the bearing lifetime, compared with the internally originating-type flaking, and the development of a long-lifetime bearing is often related to the reduction in the surface originating-type flaking. However, a carbon- or nitrogen-enriched region must be formed on the surface so as to generate a large amount of retained austenite and in turn, a quenching treatment in a special gas atmosphere, such as carburizing or carbonitriding, is required. Furthermore, an expensive alloy element such as Mo is added in many cases, because precipitation of a large amount of retained austenite gives rise to a decrease in the surface hardness most needed for a rolling bearing and this must be compensated for by a hard carbonitride.
On the other hand, toughness is in a trade-off relationship with hardness of a material, and a basic approach for enhancing the toughness is to ensure as many low-hardness regions as possible. From such a viewpoint, a carburized bearing obtained by subjecting a low/medium carbon steel to a carburizing or carbonitriding treatment to harden only the surface has been developed. However, at present, a carburized steel is used in many cases for a relatively large bearing such as bearing for iron and steel plants, and the quenching property is ensured principally by the addition of an expensive alloy element such as Ni, Mo and Cr, incurring a rise in the production cost, in addition to cumbersomeness of a heat treatment such as carburizing treatment.
Under such background, hardening only the surface of a steel material by induction heating is also proposed (see, for example, Patent Documents 1 and 2). The induction heating allows, within the single same part, a hardened layer capable of withstanding a high surface pressure to exist on the surface and an unhardened layer excellent in the toughness to exist in the inside (core part), whereby a bearing satisfying both lifetime and toughness is obtained.